This invention relates generally to photoimagable compositions, and more particularly to a solder mask having cationically polymerizable non-brominated epoxy resin system.
There are many different instances where photoimagable solder masks are used in various industrial processes. In one particular process the solder mask is applied to printed circuit board, then photolithographic techniques are employed to reveal various underlying structures on the board while masking others so that solder may be applied to the exposed structures. During the solder applying process the solder will adhere to the exposed underlying components and be prevented from adhering where the remaining material operates as a solder mask.
It is necessary that the solder mask material be formulated such that it possesses rheological properties for effective coating. Further, the solder mask must permit efficient transmission of the light or other exposing radiation so as to photolyze the photoinitiator through whatever thickness of material is applied. Also, of course, the solder mask must possess appropriate physical and chemical properties to withstand the application of the solder without significant deterioration or degradation and maintain its coverage over the portions of the board wherein solder is to be masked.
There have been many prior art proposals for different photoimagable compositions including many that use epoxies.
Examples of these are found in the following U.S. Pat. Nos.: 4,279,985; 4,548,890; 4,351,708; 4,138,255; 4,069,055; 4,250,053; 4,058,401; 4,659,649; 4,544,623; 4,684,671; 4,624,912; 4,175,963; 4,081,276; 4,693,961; and 4,442,197. All of these patents show various resins and photoinitiators for use in photoimagable compositions many of which are useful as solder masks. However none of them teach or suggest the specific composition of the present invention.
According to the present invention, an improved photoimagable cationically polymerizable epoxy based solder mask is provided that contains a non-brominated epoxy resin system and from about 0.1 to about 15 parts, by weight per 100 parts of resin system, of a cationic photoinitiator. The non-brominated epoxy resin system has solids that are comprised of from about 10% to about 80% by weight, of a polyol resin having epoxy functionality; from about 0% to about 90% by weight of a polyepoxy resin; and from about 25% to about 85% by weight of an difunctional epoxy resin. Since the photosensitive cationically polymerizable epoxy based solder mask does not contain bromine, it is particularly advantageous halogens in waste processing chemicals or in incinerated scrap circuit boards are regulated by environmental concerns. The photosensitive cationically polymerizable non-brominated epoxy based solder mask has a glass transition temperature greater than about 100xc2x0 C., preferably greater than about 110xc2x0 C. The solder mask dries to a tack-free film; thus, artwork used in the photoimaging process will not stick to the dried soldermask film.
The polyol resin which is a condensation product of epichlorohydrin and bisphenol A, has a weight average molecular weight of between about 40,000 and 130,000. The polyepoxy resin is an epoxidized multi-functional bisphenol A formaldehyde novolak resin having a weight average molecular weight of 4,000 to 10,000. The epoxidized diglycidyl ether of bisphenol A has two epoxide groups per molecule, a melting point of between about 80xc2x0 C. and about 110xc2x0 C. and a weight average molecular weight of between about 600 and 2,500. The invention also relates to a cationically polymerized solder mask.
The present invention provides a photoimagable solder mask which comprises a cationically polymerizable epoxy resin system and a photoinitiator. The solder mask is stable at 320xc2x0 C. for 30 minutes and has a glass transition temperature greater than about 100xc2x0 C., preferably greater than about 110xc2x0 C., more preferably about 120xc2x0 C. The photoimageable solder mask is applied to a substrate by conventional coating techniques such as roller coating, wound wire rod coating and curtain coating, for use as a solder mask. However, the photo solder mask can be used for other applications, such as an etch mask, permanent plating resist or protective coating. In this particular solder mask application, the photoimagable solder mask is typically coated by conventional coating techniques onto substrate to a thickness of about 0.5 to about 4.0 mils or more, dried, photoimaged and developed. The developed areas reveal the desired underlying metallized portions of the substrate wherein solder is to be applied, and the remaining solder mask material is cured and remains on the board as a solder mask during the application of solder by any conventional technique. Thus, the system of this invention must have satisfactory rheological properties for application techniques, it must be sensitive to exposure to radiation, which conventionally is in the 330 to 700 nm region, without any significant absorption to thereby allow penetration of the exposure radiation completely through the film; and it must also have the necessary physical and chemical properties to resist degradation during the soldering process.
For many applications the solder mask remains on the board after soldering. In formulating such a coating there are many competing factors that must be considered in order to provide a desired end product. No specific type of epoxy resin has been found which will satisfy all of the various requirements; however, a formulation of a combination or mixture of various epoxy resins according to this invention provides the required properties for a curtain coatable photoimagable solder mask composition.
The photosensitive epoxy solder mask comprises about 100 parts of non-brominated cationically polymerizable epoxy-resin system and about 0.1 to 15 parts by weight, of a cationic photoinitiator. The non-brominated epoxy resin system comprises between about 10% and about 80% by weight of a polyol resin of a high molecular weight and having epoxy functionality; between about 0% and about 90% by weight of a polyepoxy resin of a medium molecular weight; from about 25% to about 85% by weight of a difunctional, solid, low melting point epoxy resin. The epoxy resin system is characterized in that it does not contain brominated resins. The cationic photoinitiator is capable of initiating polymerization in the non-brominated epoxidized resin system upon exposure to actinic radiation. Preferably, the non-brominated resin system is further characterized by having an absorbance of light in the 330 to 700 nm region of less than 0.1 for a 2 mil thick film.
The Non-Brominated Epoxy Resin System
In general, the non-brominated epoxy resin system contains a phenoxy polyol resin which is a condensation product of epichlorohydrin and bisphenol A, herein also referred to as the xe2x80x9cpolyol resinxe2x80x9d. The weight average molecular weight of the polyol resin is between about 40,000 and about 130,000 preferably from about 60,000 to about 90,000 and a weight per epoxide of from about 20,000 to about 65,000, preferably about 30,000 to about 45,000. Preferably the polyol resin is a difunctional epoxy resin. A suitable resin of this type was formerly sold by Union Carbide Corporation under the trademark PKHC and is currently sold by Phenoxy Resins Incorporated, also under the trademark PKHC. This resin has an epoxide value of about 0.03 equivalents per kg, a weight per epoxide of about 37,000, and a Tg, that is, glass transition temperature, of about 98xc2x0 C.
The second resin in the system, although optional, is preferred. The second resin is an epoxidized multi-functional bisphenol A formaldehyde novolak resin with a medium range molecular weight and at least about 3 epoxy groups per molecule, also referred to herein as the xe2x80x9cpolyepoxy resinxe2x80x9d: The weight average molecular weight of the polyepoxy resin is between about 4,000 and about 10,000 preferably from about 5000 to about 7000 and the weight per epoxide is from about 180 to about 500, preferably from about 190 to about 250. A suitable resin of this type formerly sold by Hi-Tek Polymers under the Trade Mark EpiRez(copyright) SU-8, is now sold by Shell Chemical Company, Houstan Tex., under the trademark Epon(copyright) SU-8. This resin is an octa-functional resin and has an epoxide value of about 4.7 equivalents per kg, a weight per epoxide of about 215, a melting point of about 82xc2x0 C. and a Tg of about 210xc2x0 C., in its fully cured state.
The third resin in the system, which is solid at 20xc2x0 C., is a difunctional, epoxidized diglycidyl ether of bisphenol A, having a molecular weight of generally about 600 to 2,500 and preferably about 1,000 to 2,000 and a weight per epoxide of about 300 to about 1250, preferably about 500 to about 1000. This resin has a melting point of from about 80xc2x0 C. to about 110xc2x0 C. A suitable resin of this type formerly sold by High-Tek Polymers Corporation under the Trade Mark EpiRez(copyright) 522-C, is now sold by Shell Chemical Company, Houstan Tex., under the trademark Epon(copyright) 1002 F. This resin has a weight per epoxide of about 587, a weight average molecular weight of about 1174, and a melting point of about 86xc2x0 C. Other suitable epoxy resins include, for example, D.E.R. 664, D.E.R. 667, D.E.R. 668 and D.E.R. 6695 from Dow Chemical, and another resin, formerly sold by High-Tek under the trademark EpiRez(copyright) 530-C resin, which is now sold by Shell Chemical Company under the trademark Epon(copyright) 1004 F. Epi-Rez(copyright) 530-C has a melting point 100xc2x0 C. and a weight per epoxide of about 900.
It has been found generally that from about 10% to 80%, preferably 20% to 40% and more preferably 25% to 35% of the polyol resin can be used. About 0 to 90% preferably about 20% to 35% and more preferably 25% to 30% of the polyepoxy resin can be used. About 25% to 80% of the difunctional solid epoxidized bisphenol A resin is used. Preferably about 60% to about 80% of the difunctional solid epoxidized bisphenol A is desireable were no polyepoxy is used and preferably about 37% to about 47% of the difunctional solid epoxidized bisphenol A the polyepoxy resin is used.
The specific composition is selected to optimize the desired properties. For example, the polyol resin controls the rheology of the coated materials while the difunctional solid epoxy and the polyepoxy impart fast photo speed, improved resolution and glass transition temperature. In selecting the specific amount of each resin it will of course be understood that by increasing the concentration of any one of the resins, this would also increase the respective property associated with it; however, increasing the concentration of any one of the specific resins would require a decrease in the concentration of one or both of the remaining resins which would result in a decrease of the function of the specific properties associated therewith. A decrease in any of the resins below the broad range percentages indicated above would result in properties which are unacceptable for the specific purpose of high resolution photoimagable solder mask material.
As the polyol resin amount is decreased, the resulting rheology results in a reduced coverage when the material is applied and the resulting material is extremely brittle. Varying the multi-functional epoxy resin concentration or the solid difunctional epoxy resin concentration affects both the resolution and photospeed. Also, increasing the solid difunctional epoxy resin concentration increases the compatability of the components of the resin system. Thus, in balancing the formulation of the material, all of these requirements must be considered and the end product optimized to give satisfactory properties for the result desired.
The Photoinitiator
The cationic photoinitiator is capable of initiating polymerization of the non-brominated epoxy resin system upon exposure to actinic radiation. A photoinitiator-such as various sulfonium salts, iodonium salts, and ferrocene salts is added to the resin system for providing the proper photoresponse to actinic radiation. Since the resin system is cationically photocurable, the photoinitiator must be capable of causing cationic polymerization of the resin upon exposure to radiation. One particularly desirable photoinitiator is a complex triarylsulfonium hexafluoroantimonate salt formerly sold by General Electric Company under the trademark UVE 1014, and presently sold by Union Carbide under the trademark Cyracure UVI-6974.
Other photoinitiators such triphenylsulfonium hexafluorophosphate sold by General Electric Co. under the Trade Mark UVE 1016, and diphenyliodonium hexafluoroantimate may be used.
From about 0.1 and about 15 parts by weight of the photoinitiator based on 100 parts of the resin system are normally what is required. (It is conventional practice in the art of photoimaging to designate the percentage of the components of the resin system to add up to 100% and to designate the additions or additives thereto in values of parts by weight based on 100 parts of the resin system, and this convention is adopted herein.)
Solvent
To obtain the necessary viscosity for applying the photoimagable solder mask composition, the photoimageable solder mask composition contains a solvent. The amount of solvent depends upon the coating method. For curtain coating the photoimageable solder mask composition preferably contains about 40% solids in the solvent. For wire rod coating the photoimageable solder mask composition preferably contains about 30% to about 50% solids more preferably about 40% solids in the solvent. The preferred solvent is propylene glycol monomethyl ether acetate (PGMEA). Other moderately polar solvents can be used such as propylene glycol monomethyl ether, 2-methoxyethanol, 2-methoxyethyl acetate, 2-ethxyethyl acetate, N-methyl pyrrolidone, propylene carbonate, or gammabutyrolactone.
Optional Ingredients
For some applications, certain additives to the formulation may be desired. Optionally a photosensitizer such as anthracene or its derivatives or perylene or its derivatives can also be added which may increase the response speed of photoimaging and/or the wavelength sensitivity. Optionally up to about 10 parts by weight of a photosensitizer, based on 100 parts of the resin system, can be used. A fluorescent or color dye may be added for inspection or cosmetic purposes. For some coating applications, it may be desirable to use a surfactant, e.g. Fluorad FC 430 sold by 3M Corp. It will, of course, be understood that the additive must not significantly degrade the other properties of the coating.